The hepatitis C virus (HCV) nonstructural protein NS5A is critical for viral genome replication and is thought to interact directly with both the RNA-dependent RNA polymerase, NS5B, and viral RNA. NS5A consists of three domains which have, as yet, undefined roles in viral replication and assembly. In order to define the regions that mediate the interaction with RNA, specifically the HCV 3 untranslated region (UTR) positive-strand RNA, constructs of different domain combinations were cloned, bacterially expressed, and purified to homogeneity. Each of these purified proteins was probed for its ability to interact with the 3 UTR RNA using filter binding and gel electrophoretic mobility shift assays, revealing differences in their RNA binding efficiencies and affinities. A specific interaction between domains I and II of NS5A and the 3 UTR RNA was identified, suggesting that these are the RNA binding domains of NS5A. Domain III showed low in vitro RNA binding capacity. Filter binding and competition analyses identified differences between NS5A and NS5B in their specificities for defined regions of the 3 UTR. The preference of NS5A, in contrast to NS5B, for the polypyrimidine tract highlights an aspect of 3 UTR RNA recognition by NS5A which may play a role in the control or enhancement of HCV genome replication.

A number of new ruthenium compounds have been synthesised, isolated and characterised, which exhibit excellent cytotoxicity against a number of different human tumour cell lines including a defined cisplatin resistant cell line and colon cancer cell lines. Addition of hydrophobic groups to the ruthenium molecules has a positive effect on the cytotoxicity values. Evidence is provided that, after incubation of a ruthenium compound with a 46 mer oligonucleotide duplex and subsequent nuclease treatment, ruthenium is bound to a guanine residue.

Interactions between the cAMP receptor protein (CRP) and the carboxy-terminal regulatory domain (CTD) of Escherichia coli RNA polymerase ␣ subunit were analyzed at promoters carrying tandem DNA sites for CRP binding using a chemical nuclease covalently attached to ␣. Each CRP dimer was found to direct the positioning of one of the two ␣ subunit CTDs. Thus, the function of RNA polymerase may be subject to regulation through protein-protein interactions between the two ␣ subunits and two different species of transcription factors.The RNA polymerase holoenzyme of Escherichia coli is composed of core enzyme with subunit structure ␣ 2 ␤␤Ј, responsible for RNA polymerization, and one of multiple species of subunit, responsible for promoter recognition. Promoter selectivity of the holoenzyme is modulated by direct or indirect interaction with many transcription factors, resulting in switching of the global pattern of gene transcription according to the environment. The best-characterized target on the RNA polymerase involved in molecular communication with transcription factors is the ␣ subunit carboxy-terminal domain (CTD) that contains the contact sites for class I transcription factors. The ␣ subunit, consisting of 329 amino acid residues, is composed of two structural domains, each responsible for distinct functions (1-3) and each forming independent structural domains connected by a protease-sensitive flexible linker (4-6). The amino (N)-terminal domain from residues 20 to 235 plays a key role in RNA polymerase assembly by providing the contact surface for ␣ dimerization and binding of ␤ and ␤Ј subunits (7-10), whereas the CTD from residues 235 to 329 plays a regulatory role by providing the contact surfaces for trans-acting protein factors and cis-acting DNA elements (11)(12)(13)(14).Whereas the regulation of many E. coli promoters involves a single factor, some promoters are regulated by two or more transcription factors, and such coregulation systems involving multiple species of transcription factors can couple gene expression to diverse environmental conditions. Knowledge of the molecular mechanism of prokaryotic transcription regulation involving more than two factors would contribute much to understanding of the events carried out in eukaryotes, because the regulation of gene transcription in eukaryotes generally involves the action of multiple transcription factors. To gain insight into this problem, we analyzed interactions between RNA polymerase and cAMP receptor protein (CRP) dimers on promoters carrying tandem CRP-binding sites at various positions relative to the transcription start site. A set of promoters was constructed carrying one DNA site for CRP centered at position Ϫ41.5 upstream from the transcription start point and a second DNA site for CRP located further upstream (refs. 15

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DNAase I footprinting has been used to study open complexes between Escherichia coli RNA polymerase and the galactose operon P1 promoter, both in the absence and the presence of CRP (the cyclic AMP receptor protein, a transcription activator). From the effects of deletion of the C-terminal part of the RNA polymerase alpha subunit, we deduce that alpha binds at the upstream end of both the binary RNA polymerase-galP1 and ternary RNA polymerase-CRP-galP1 complexes. Disruption of the alpha-upstream contact suppresses open complex formation at galP1 at lower temperatures. In ternary RNA polymerase-CRP-galP1 complexes, alpha appears to make direct contact with Activating Region 1 in CRP. DNAase I footprinting has been used to detect and quantify interactions between purified alpha and CRP bound at galP1.

SummaryMelR is a melibiose-triggered transcription activator that belongs to the AraC family of transcription factors. Using purified Escherichia coli RNA polymerase and a cloned DNA fragment carrying the entire melibiose operon intergenic region, we have demonstrated in vitro open complex formation and activation of transcription initiation at the melAB promoter. This activation is dependent on MelR and melibiose. These studies also show that the cyclic AMP receptor protein (CRP) interacts with the melAB promoter and increases MelR-dependent transcription activation. DNAase

The Escherichia coli melAB promoter is co-dependent upon two transcription activators, MelR and the cyclic AMP receptor protein, CRP. In this study we demonstrate positive co-operativity between the binding of MelR and CRP at the melAB promoter, which provides a simple mechanism for its co-dependence. MelR binds to four sites, centred at positions ±42.5, ±62.5, ±100.5 and ±120.5 relative to the melAB transcription start point. When MelR is pre-bound, CRP is able to bind to a target located between MelR at positions ±62.5 and ±100.5. This increases the occupation of the two downstream sites for MelR, which is essential for transcription activation. We have identi®ed residues within activating region 1 (AR1) of CRP that are important in transcription activation of the melAB promoter. At simple CRP-dependent promoters, the surface of CRP containing these residues is involved in contacting the RNA polymerase a subunit. Our results show that, at the melAB promoter, the surface of CRP containing AR1 contacts MelR rather than RNA polymerase. Thus, MelR and CRP activate transcription by a novel mechanism in which they bind cooperatively to adjacent sites and form a bacterial enhanceosome.

Hydroxyl radical footprinting has been used to study different open complexes between Escherichia coli RNA polymerase and the galactose operon regulatory region, which contains two overlapping promoters, P1 and P2. Complexes at P1 were studied by exploiting a P2- mutant and complexes at P2 were studied with a P1-mutant. We have identified the precise location of alpha binding in both binary RNA polymerase-galP1 and RNA polymerase-P2 complexes from the effects of deletion of the C-terminal domain of the RNA polymerase alpha subunit: alpha binds to different sites at the upstream end of each complex. Transcription initiation at galP1 can be activated by the cyclic AMP receptor protein (CRP). Addition of CRP to the RNA polymerase-galP1 complex displaces the C-terminal domain of alpha, which then binds to a different site upstream of CRP in the ternary CRP-RNA polymerase-galP1 complex. Thus, the C-terminal domain of alpha can occupy three different sites at the gal operon regulatory region. We have also examined the effect of disrupting the Activating Region of CRP on interactions between CRP and the C-terminal domain of alpha in ternary CRP-RNA polymerase-galP1 complexes. Footprinting experiments show that these substitutions interfere with the contact between CRP and alpha but do not affect the position of alpha binding to its site upstream of bound CRP.